1. Field of the Invention
The invention relates generally to a temperature sensing device and more particularly to a device that provides a warning when a thyristor within the device detects an increase in temperature in an item to which the temperature sensing device is in close proximity.
2. Description of the Prior Art
There are many devices used in the electrical safety area, such as fuses, circuit breakers and circuit protectors, that function when an excessive current situation develops in a main electrical line. Similarly, there are many types of smoke detectors available that sound a warning once smoke has been generated. The present invention comprises a small, inexpensive semiconductor circuit that can be placed near an item such as an electrical switch, socket or plug. The invention gives a warning when a temperature increase near the item occurs thus allowing a fire or other dangerous condition to be avoided.
The substantial negative temperature-resistance coefficient of some semiconductor materials has been utilized in the prior art to create temperature dependent switching devices. For example, U.S. Pat. No. 3,343,004, issued to Ovishinsky on Sept. 19, 1967 for a "Heat Responsive Control System", discloses a current controlling device comprising a temperature sensitive semiconductor component connected in series with a load resistance. When a temperature increase occurs, the blocking resistance of the semiconductor component drops permitting current to flow in the circuit.
Similarly, systems have been reported for sounding an alarm in response to an increase in temperature. Representative examples of the prior art include U.S. Pat. No. 4,550,313, issued to Kimura on Oct. 29, 1985 for a "Fire Detecting System", which discloses a general circuit that utilizes a comparator to produce an alarm signal when the output of a sensor exceeds a reference voltage. U.S. Pat. No. 4,538,137, issued to Kimura on Aug. 27, 1985 for a "Fire Detector", discloses a circuit in which the output voltage of the sensor activates a Zener diode and a thyristor which in turn alters the frequency at which a capacitor is discharged.
U.S. Pat. No. 4,315,256, issued to Dennis on Feb. 9, 1982 for a "Chimney Fire Detector", discloses a temperature sensing circuit in which the sensor comprises a pair of insulated twisted wires connected to a thyristor which is connected in series to an alarm. A temperature increase melts the insulation of the wires allowing the wires to touch and complete the circuit. U.S. Pat. No. 3,728,702, issued to Miyamoto, et al on Apr. 17, 1973 for a "Temperature Alarm Utilizing Paired Positive and Negative Coefficient Thermistors", discloses the use of series connected thermistors to generate a signal that operates a Schmitt circuit.
None of the prior art discloses a circuit that can be conveniently utilized to provide an early warning that a dangerous temperature condition is developing in an item.